Add test for soong/testing/test_spec.

This CL adds test for test_spec.go and all_test_specs.go (singleton).

Bug: 296873595
Test: m nothing --no-skip-soong-tests -j96

Change-Id: I5010c68512e75d1b9a337c02da86faac15e376fe
diff --git a/testing/Android.bp b/testing/Android.bp
index 85e6a8b..18dccb3 100644
--- a/testing/Android.bp
+++ b/testing/Android.bp
@@ -15,6 +15,7 @@
         "all_test_specs.go",
         "test_spec.go",
         "init.go",
+        "test.go",
     ],
     pluginFor: ["soong_build"],
 }